The invention is directed to a drilling tool bit with a carrier member and at least one cutter member where its radial dimension diminishes continuously in the direction opposite to the drilling direction and the cutter member extends radially outwardly and axially from the circumferential surface of the carrier member.
Drilling tool bits of the above type serve for drilling rock, concrete, masonry and similar materials. The carrier members can be differently shaped. It is common, however, to all drilling tool bits that they have one or more cutter members located at the leading end of the bit wherein these cutter members are formed of wear-resistant material, such as hard metal or alloy, mechanically resistant metal, or the like.
The carrier member can be in the form of an axially extending shank with a helix extending around its circumferential surface, and the helix can be molded as one piece with the shank or it can be connected to it. Cutter members project outwardly from the carrier member radially as well as axially in the drilling direction and as an example, a single cutter member can be used for smaller diameter carrier members and several cutter members can be used for larger diameter carrier members. As an example, a drilling tool bit, in particular for a larger diameter, is disclosed in DE-PS 25 43 578.
Further, it is possible, especially for drilling boreholes of larger diameter, to use a hollow annular carrier member. In such an arrangement, the cutter members project axially as well as radially outwardly in the drilling direction from the leading end of the carrier member, with the cutter members being spaced angularly apart around the circumference of the carrier member. The length of the cutter members in the drilling direction must not be less than a minimum dimension, so that a sufficiently large contact surface exists between the cutter member and the carrier member, such a hollow annular drilling bit is known in DE-PS 24 19 548.
It is common in such cutter members that the radial dimension diminishes continuously opposite to the drilling direction, as the previously mentioned drilling tool bit demonstrates. Accordingly, in the case of a hollow annular drilling bit used as a carrier member, the width of the cutter members measured in the radial direction opposite to the drilling direction reduces continuously towards the inner as well as the outer surface of the carrier member. By such reduction in the radial dimension of the cutter members, it is possible to reduce the friction of the cutter members against the borehole wall surface.
As long as the drilling bits are new, the reduction in friction is achieved. The longer the drill bits are in use, however, the more the radially measured dimension of the cutter members approaches a constant dimension due to wear. Accordingly, the surface of the cutter members producing friction opposite to the drilling direction in contact with the wall surface of the bore hole being drilled increases and the efficiency of the drill bit is considerably reduced because of the continuous increase in friction. In the extreme case, the surface of the cutter member producing friction against the wall surface of the borehole being formed extends for its entire axial length, that is, for the length of the cutter members embedded in the carrier member. As a result, this leads to an overload on the drilling bit and causes jamming of the bit in the borehole being formed.
These problems are further worsened in the case of solid drilling bits with more than one cutter member which have come into increased use in recent years. Such drilling bits produce circular holes with closer tolerances than has been known in the past when utilizing drilling bits equipped with a single cutter member. The jamming effect due to the wear phenomenon explained above is particularly acute, so that the pertinent problems are intensified. This set of problems appears in an equally undesirable manner also with drilling bits whose carrier member is configured as a hollow annular drill bit with several cutter members, since such drilling bits produce boreholes also with close tolerances.